21 That all may understand more exactly the teachings which the selected texts of the Old and New Testament furnish concerning this devotion, they must clearly understand the reasons why the Church gives the highest form of worship to the Heart of the divine Redeemer. As you well know, venerable brethren, the reasons are two in number. The first, which applies also to the other sacred members of the Body of Jesus Christ, rests on that principle whereby we recognize that His Heart, the noblest part of human nature, is hypostatically united to the Person of the divine Word. Consequently, there must be paid to it that worship of adoration with which the Church honors the Person of the Incarnate Son of God Himself. We are dealing here with an article of faith, for it has been solemnly defined in the general Council of Ephesus and the second Council of Constantinople.(15)
Source: Haurietis Aquas (Vatican.va)